Styrene-diene block copolymers have been formulated in the past to produce a number of types of adhesive compositions. The basic patent in this field, Harlan, U.S. Pat. No. 3,239,478, shows combinations of these block copolymers with tackifying resins and paraffinic extending oils to produce a wide spectrum of adhesives. However, one of the serious limitations of these adhesive compositions is their relatively low service temperatures. Generally, the highest temperature at which these styrene-diene block copolymers retain useful properties and act like a vulcanized rubber is limited by the softening temperature (Tg) of the styrene end block. Depending upon the molecular weight of the end blocks and the load, these block copolymers can begin to significantly creep or flow at 120.degree.-180.degree. F. For a number of applications it would be very advantageous to have higher service temperatures. For example, these adhesives would be useful in paint shops if the masking tapes produced had a 225.degree.-250.degree. F service temperature.
One means to improve the service temperature of these styrene-diene block copolymers is to mechanically mix a resin with the block copolymer which will increase the softening temperature of the styrene end blocks. However, the resins now currently available for this purpose are limited to service temperature increases of only about 30.degree. F or less at 30 phr (parts per hundred rubber) loading.
One possible resin not previously disclosed for use in adhesive compositions is a polyphenylene ether resin. These polyphenylene ethers are well known and are described in numerous publications including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358. These polymers are high performance engineering thermoplastics having high molecular weights and possessing high melt viscosities and softening points -- i.e., Tg equals 210.degree. C, &gt;10.sup.4 poise at 315.degree. C. However, the relatively high melt viscosities and softening points are a serious disadvantage in blending with the subject styrene-diene block copolymers. While it has been shown that blends containing the polyphenylene ether resins can be prepared using solution techniques, melt processing before this has been commercially unattractive because of the high temperature required to soften the resin and the problems associated therewith such as instability and discoloration. Specifically, the subject styrene-diene block copolymers are unstable and cross link at the 275.degree. C temperature required for direct melt blending with the commercially available polyphenylene ether resins.
It is known in the art that properties of the polyphenylene ether resins can be materially altered by blending them with other resins. For example, one method for improving the melt processability of the polyphenylene ethers is disclosed in U.S. Pat. No. 3,379,792. According to this patent, flow properties of the polyphenylene ethers are improved by blending with from about 0.1 to 25 parts by weight of a polyamide. In another patent, U.S. Pat. No. 3,361,851, a polyphenylene ether composition comprising a polyphenylene ether blended with a polyolefin is disclosed. The polyolefin is added to improve impact strength and resistance to aggressive solvents. In a third patent, Cizek, U.S. Pat. No. 3,383,435, there are provided means for simultaneously improving the melt processability of the polyphenylene ether resins while simultaneously up-grading many properties of the polystyrene homopolymer and random copolymer resins. The invention of the Cizek patent is based upon the discovery that the polyphenylene ether resins and such polystyrene resins, including rubber modified polystyrene resins, are combinable in all proportions and result in compositions having many properties improved over those of either of the components. However, the polystyrene resins of Cizek having high molecular weight are incompatible with the end blocks of the subject styrene-diene block copolymers.
In Kambour, U.S. Pat. No. 3,639,508, "liquid phase blending" of a polyphenylene ether resin and a monovinyl aromatic-diene block copolymer is disclosed. Liquid phase blending is defined to include both solution blending and melt blending. However, this melt blending according to Kambour is effective at temperatures of 100.degree. C over the glass transition temperature of the polyphenylene ether resin or, more particularly, over a range of 275.degree. C to 310.degree. C. As noted above, these melt blending temperatures are typically above the gelling temperature of the styrene-diene block copolymer. Such gelling can be tolerated when the styrene-diene block copolymer is only an impact modifier, a minor component. However, when the styrene-diene rubber is the major component, gelling dramatically decreases the properties of the blend. Accordingly, in all examples of Kambour employing a styrene-diene block copolymer, only solution blending was employed. See also U.S. Pat. Nos. 3,663,661; 3,835,200; and 3,994,856, which teach that high temperature melt blending has many disadvantages.
Other blending schemes, such as that disclosed in Lauchlan et al., U.S. Pat. No. 3,660,531, also have their built-in disadvantages due to the necessary presence of a high molecular styrene resin.
A new melt blending process has been discovered that permits the melt blending of a polyphenylene ether resin and a styrene-diene block copolymer without the use of a solvent and at temperatures that do not result in damage to the block copolymer. Further, novel adhesive compositions have been discovered that possess improved high temperature performance characteristics.